10 Questions and Answers on Fetal Ultrasound Examination in the Second Trimester

Ultrasonography is widely used for prenatal assessment of fetal growth, anatomy, and management of multiple pregnancies. Second-trimester ultrasound is primarily used to evaluate fetal anatomy and can detect the most clinically significant structural abnormalities. There are significant differences in detection rates among medical centers and among examiners. 1. Who should receive mid-trimester fetal ultrasound examination? It is recommended that all pregnant women receive routine fetal ultrasound examination in the second trimester. Standardized routine fetal ultrasound examination in the second trimester can detect malformations and abnormalities that are not obvious in early pregnancy. Second-trimester fetal ultrasound can detect more abnormalities and reduce the cost of detecting each abnormality. Through statistics on birth defects every year, it is found that the detection rate of congenital heart defects has increased significantly, indicating that these tests are effective. If abnormalities are discovered or suspected during early pregnancy examinations, they should be immediately referred to relevant experts for evaluation and consultation without waiting for a second-trimester fetal ultrasound scan. Detailed ultrasound examination can be performed thereafter as needed. 2. When should we start routine fetal ultrasound examination in the second trimester? It is recommended to conduct routine fetal ultrasound examination in the second trimester between 18 and 24 weeks of pregnancy. To ensure sufficient time for consultation and further inspection, a balance should be struck between inspection time and detection rate. 3. Is prenatal ultrasound examination safe? Prenatal ultrasound examination is safe. The ALARA principle (as low as reasonably achievable) should be followed, fetal exposure time should be reduced as much as possible, and the lowest possible output power should be used to obtain the information required for diagnosis, not just for parent entertainment purposes. Ultrasound equipment, probes, and gels should be properly maintained and handled to provide a safe environment for patients and staff. While prenatal ultrasound can provide beautiful, memorable images of your fetus, it should not be used solely for entertainment purposes. 4. When is pulse wave Doppler technology needed? Pulse wave Doppler technology is not included in routine ultrasound examinations during the second trimester. There is insufficient evidence to support the use of color Doppler imaging of the uterine or umbilical arteries in low-risk pregnant women. Color flow Doppler imaging is helpful in examining the number of blood vessels in the fetal heart and umbilical cord and the amount of amniotic fluid. 5. How does ultrasound assess amniotic fluid volume? A semi-quantitative method is used to assess amniotic fluid volume. Amniotic fluid volume can be assessed subjectively, defined as \”normal\” or \”abnormal\” (decreased or increased), or semi-quantitatively, by measuring the deepest vertical pocket (DVP) or amniotic fluid index (AFI). The probe is perpendicular to the skin of the pregnant woman\’s abdomen, and the edges of the upper and lower boundaries of each quadrant are clearly defined. The maximum depth of amniotic fluid without the umbilical cord and fetal body is measured, and color Doppler is used to check where the umbilical cord is unclear. DVP ≤ 2.0cm is considered to be a decrease in amniotic fluid, DVP > 2.0cm and ≤ 8.0cm is considered normal amniotic fluid volume, DVP > 8cm is considered to be an increase in amniotic fluid volume, and relative gestational age reference values ​​can also be applied. The amniotic fluid index (AFI) may be more suitable for assessing polyhydramnios, whereas the maximumThe vertical depth (DVP) may be more appropriate for the assessment of oligohydramnios. 6. To estimate fetal weight (EFW), most clinical practices currently recommend the use of the Hadlock-3 formula (HC, AC, FL). This formula appears to be the most stable mathematically, and the deviation of the estimated fetal weight from the expected mean for the corresponding gestational age should be expressed as percentiles (or Z-scores), with the selected reference standard noted in the report. Reference values ​​for fetal biological measurement parameters derived from regular, prospective, truly population-based studies derived from studies with minimal methodological bias should be favored. The 2016 Delphi consensus definition of fetal growth restriction (FGR) should be used. Ultrasound measurement during the second trimester detects abnormalities in fetal growth and development, and serves as a basic parameter for subsequent evaluation of fetal growth abnormalities. The Hadlock-3 formula provides the best estimate of fetal weight in a large study cohort, considering growth assessment methods available for all fetuses, including fetuses suspected of being undersized or larger. In the early stages of pregnancy, it is still unclear to what extent the difference from normal values ​​requires further examination (such as follow-up ultrasound examination to assess fetal growth or fetal chromosome analysis, etc.). Recent research shows that in comparative models, estimated fetal weight (EFW) as early as mid-pregnancy can be used to predict subsequent risk of small for gestational age. Additional measurements reflect fetal growth status, taken at least 3 weeks apart from the previous examination (2022 Update), usually reported to reflect deviations from the expected mean for the corresponding gestational age. It is best expressed as a reference value range or Z value. Using Z value can monitor serious growth abnormalities and facilitate quality control. The selected reference standard should be noted in the report. Examiners should be aware of national or local recommended charts and select appropriate fetal biometry charts. If growth abnormalities are suspected, it is recommended to use the 2016 Delphi-based consensus diagnostic criteria for fetal growth restriction for evaluation. Abnormal fetal umbilical artery Doppler and/or maternal hypertension or preeclampsia should prompt emergency referral (updated 2022). 7. Measure the length of the femur (Femur Length, FL). The ossified metaphyses at both ends of the femur are clearly visible. The longest straight-line distance between ossified metaphyses was measured. The rulers were placed on the ossified metaphyseal edges at both ends of the femur, excluding the distal epiphysis, to avoid including triangular raised burr artifacts, which would extend the length of the femoral edge and cause measurement errors. Consistent with the technology used to establish the corresponding reference value range, the angle between the incident angle of the sound beam and the femur is usually between 45° and 90°. Technical improvements in modern ultrasound instruments have reduced the beam width and reduced the impact on lateral measurements. Clinical In practice, measurement parameter tables from recent years should be used, as old parameter tables may cause FL values ​​to be overestimated. 8. How to measure the biparietal diameter (BPD). The anatomical position is a cross section at the level of the thalamus of the fetal head; the ideal ultrasound incident angle is 90° between the sound beam and the midline of the brain, but slight changes are allowed; 1.The two cerebral hemispheres are symmetrical; the midline echo (falx cerebri) is continuous, interrupted only by the septum pellucidum in the middle; the cerebellum is not visible. Four-dimensional color ultrasound mainly examines the fetus. There are many ways to place the ruler (such as \”outer edge to inner edge\” and \”outer edge to outer edge\”, etc.). The two rulers should be placed at the widest part of the skull, and the connecting line is perpendicular to the midline of the brain. . The measurement method is consistent with that used in establishing the reference value range. The cephalic index is the ratio of the maximum width (BPD) of the fetal head to the maximum length (Occipital-Frontal Diameter, OFD). This value can be used to describe the shape characteristics of the fetal head. Abnormal head shapes (such as brachycephaly or dolichocephaly) Cephaly) may be associated with certain syndromes, oligohydramnios, or breech presentation. Predicting gestational age using BPD is not accurate; for these cases, application of HC would be more accurate. Recent evidence suggests placement of the ruler from outer edge to outer edge Easier standardization, high reproducibility and easy image quality control (updated in 2022). 9. How to measure head circumference (HC)? The anatomical location is consistent with the anatomical structure of the BPD plane. The placement of the ruler for measuring HC is consistent with the reference value The placement of the middle ruler is consistent. If the ultrasound equipment has an elliptical measurement function, the elliptical measurement ruler can be placed on the outer edge of the skull to directly measure HC. Or HC can be calculated based on BPD and OFD, as follows: BPD is measured using external to The internal measurement technique is as described in \”Biparietal Diameter\”, while the OFD is placed with a ruler at the center of the frontal and occipital bones on both sides. HC is then calculated by the formula HC=1.62×(BPD+OFD). Recent Evidence It shows that the placement of the ruler from outside to outside is easier to standardize, has higher repeatability, and facilitates image quality control (updated in 2022). 10. Can fetal genitals be examined? Observing external genitalia to determine fetal gender is not a routine ultrasound examination in the second trimester Content. The external genitalia should be checked for normal appearance. Gender should be reported only when parents request to know the gender due to medical necessity and in accordance with local regulations and practice guidelines.